1. Field of the Invention
The present invention relates to a method and device for analyzing a gas in a process chamber such as a sputtering or reactive ion etching chamber.
2. Description of the Related Art
In the development of electronic devices involving semiconductor integrated circuits, vacuum devices have become essential to the process of manufacturing such electronic devices, which comprises a step of treating a semiconductor in a vacuum chamber or in a gas atmosphere formed by first evacuating a chamber and then introducing a gas into the chamber.
In such various devices utilizing a special atmosphere, including a vacuum atmosphere, the degree of the vacuum in a process chamber must be extremely high (a pressure less than 10.sup.-10 Torr) , compared to a usual high vacuum, and this requirement is essential to the obtaining of a semiconductor device having a greater efficiency, a higher density, and a higher quality. Further, an improvement of the quality of an atmosphere in a process chamber and an improvement of the quality of vacuum in the chamber, as a basis of the atmosphere, is critical.
One essential basic factor in the improvement of the vacuum quality is, of course, a gas-tight sealing of the vacuum system from the outside; another is the efficiency and precision of a measurement of the atmosphere in a vacuum system, particularly in a process chamber. Note, the measurement of the atmosphere in a process chamber involves a gas analysis and a gas leakage detection.
An ionization vacuum gauge is known as a means for measuring an atmospheric pressure or a degree of vacuum, but the ionization vacuum gauge cannot analyze a gas in a chamber.
Currently, in addition to a process in a vacuum chamber, a gas atmosphere is often used in another process, for example, a reactive ion etching or a chemical vapor deposition (CVD), and a mass spectrometer is often used for an analysis of such a gas atmosphere. A mass spectrometer is an analyzer utilizing an electromagnetic interaction to detect atoms and molecules by the mass of ions thereof, and is available in several types; a residual gas analyser: RGA (qued pole mas filter; Q-mass) is often used.
FIG. 1 illustrates a gas analysis system using a mass spectrometer. In this figure, 1 denotes a process chamber, 2 an evacuating means, 3 a mass spectrometer, and 4 a valve for introducing a gas into and controlling the atmosphere in the process chamber 1. The inside of the process chamber 1 is originally open to the surrounding air, at a pressure of 760 Torr, and is first evacuated by a roughing vacuum pump such as an oil-sealed rotary vacuum pump to, for example, 10.sup.-3 -10.sup.-4 Torr. Then, the roughing vacuum pump is replaced by a high degree vacuum pump, such as an oil or mercury diffusion vacuum pump or various ion pumps, to obtain a high vacuum, e.g., 10.sup.-5 Torr or 10.sup.-8 Torr, or even an ultra high vacuum higher than 10.sup.-10 Torr (a pressure lower than 10.sup.-10 Torr). After obtaining such an ultra high vacuum, a process gas is introduced through the valve 4 into the process chamber 1 and a process treatment is carried out at a predetermined degree of vacuum.
FIG. 2 shows a general relationship between the nitrogen gas partial pressure analyzed by the mass spectrometer and the degree of vacuum in the process chamber, as obtained by the system shown in FIG. 1. Namely, an argon gas mixed with a nitrogen gas at a content of 0%, A.sub.1 %, or A.sub.2 % (A.sub.1 &lt;A.sub.2) (nitrogen leakage 0%, A.sub.1 %, A.sub.2 %) was introduced through the valve 4 and the nitrogen partial pressure, expressed as a relative value, was measured by the mass spectrometer 3 while the degree of vacuum (in Torr unit) in the process chamber was varied by varying a flow rate of the argon gas. It can be seen from FIG. 2 that, as the degree of vacuum in the process chamber 1 is reduced (pressure increased), the measured nitrogen partial pressures for the nitrogen contents 0%, A.sub.1 % and A.sub.2 % begin to reduce from predetermined degrees of vacuum points, respectively, and the reduction of the partial pressure of the nitrogen content A.sub.2 % begins first, followed by that of A.sub.1 %, and finally, that of 0%. The relationship shown in FIG. 2 is the essential characteristic of a mass spectrometer, and therefore, it is generally considered that a mass spectrometer should not be used at a pressure higher than about 10.sup.-3 Torr, and preferably, is used at a pressure lower than around 10.sup.-4 Torr, more preferably lower than around 10.sup.-5 Torr. This is because a mass spectrometer analyzes atoms and molecules by utilizing a movement of ions of the atoms and molecules in a high vacuum (a low pressure).
Nevertheless, a process such as sputtering or reactive ion etching usually must be carried out at a degree of vacuum of about 10.sup.-1 to 10.sup.-3 Torr, and therefore, a mass spectrometer is not suitable for an analysis of a gas in such a process chamber, i.e., at a low degree of vacuum of about 10.sup.-1 to 10.sup.-3 Torr.
To enable the use of a mass spectrometer for analyzing a gas in a process chamber, a method is known of increasing the degree of vacuum around a mass spectrometer to be more than that in the process chamber (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 61-130485).
FIG. 3 illustrates such a prior art system for analyzing a gas by a mass spectrometer. In this figure, separate vacuum means 21 and 22, independent of the vacuum means 2 for evacuating the process chamber 1, are provided to evacuate the gas around the mass spectrometer 3. Namely, a flow rate control valve (a pressure difference of, e.g., 1/1000) 5 is provided in a line from the process chamber 1 to the mass spectrometer 3 and a line is branched from a portion between the valve 5 and the mass spectrometer 3 and is connected to higher and lower degree vacuum pumps 21 and 22, whereby while the degree of vacuum in the process chamber 1 is, for example, 10.sup.-2 Torr or 10.sup.-3 Torr, the degree of vacuum at the mass spectrometer 3 can be, for example, 10.sup.-5 Torr or 10.sup.-6 Torr. Here, the higher degree vacuum pump 21 is a turbo molecular drag pump having a turbine wing comprised of a rotor and a stator, and the lower degree vacuum pump 22 is an oil rotary vacuum pump, whereby the mass spectrometer 3 can be operated in an atmosphere having a required high degree of vacuum (a low pressure).
FIG. 4 shows a relationship between the nitrogen gas partial pressure analyzed by the mass spectrometer and the degree of vacuum in the process chamber, in a system as shown in FIG. 3. This was also obtained by introducing an argon gas mixed with a nitrogen gas at a content of 0%, A.sub.1 %, or A.sub.2 % (A.sub.1 &lt;A.sub.2), while the vacuum pressure in the process chamber 1 was varied by varying a flow rate of the argon gas. The nitrogen partial pressure was analyzed by the mass spectrometer 3 at various degrees of vacuum Torr unit) of the process chamber 1, and expressed at a relative value. It can be seen from FIG. 4 that, when the pressure in the process chamber is increased from 10.sup.-4 Torr to 10.sup.-1 Torr (a degree of vacuum of from 10.sup.-4 Torr to 10.sup.-4 Torr), although the pressure for the mass spectrometer is then sufficiently low, at 10.sup.-7 Torr to 10.sup.-4 Torr, the nitrogen partial pressure at the zero nitrogen leakage is increased and the difference between the measured nitrogen partial pressures becomes smaller. FIG. 5 shows the relationship between the nitrogen gas partial pressure analyzed by the mass spectrometer and the degree of vacuum around the mass spectrometer, in a system as shown in FIG. 3, and this relationship corresponds to that in FIG. 4 with a pressure difference of 10.sup.-3 Torr between the process chamber and the mass spectrometer. This suggests that, when the degree of vacuum in the process chamber 1 is lower than 10.sup.-4 Torr or less (a pressure higher than 10.sup.-4 Torr), the nitrogen contents of the gases in the process chamber 1 and in the mass spectrometer 3 are not equivalent, and thus that the analysis of a gas in the process chamber 1 is not reliable within that pressure range. This unreliability of an analysis in the system as shown in FIG. 3 is different from the characteristic of the mass spectrometer shown in FIG. 2, and is derived from the system structure.
The object of the present invention is to provide a method and device for correctly analyzing a gas in a process chamber having a low degree of vacuum, e.g., 10.sup.-4 Torr or lower (a pressure of 10.sup.-4 Torr or higher) or even 10.sup.-3 Torr or lower (a pressure of 10.sup.-3 Torr or higher), by using a mass spectrometer.